The invention is in the field of digital signal processing for multi-carrier systems, particularly OFDM (orthogonal frequency division multiplex) transmission systems. In these, the transmission band is divided into a number N of subcarriers and each subcarrier is occupied with a modulation (PSK, QAM). In the transmitter, the vector of all N modulated subcarriers of an OFDM symbol present in the frequency domain is transformed into the time domain by means of a fast Fourier transform (FFT). In the receiver, the N time signal samples of an OFDM symbol are transformed back into the frequency domain by means of an FFT and demodulated there.
Whereas continuous OFDM data streams are transmitted in broadcasting (DAB, DVB-T), the transmission in the systems forming the basis here (e.g. WLAN) takes place by means of OFDM data packets of variable length which arrive at the receiver at unknown or inaccurately known times. The receiver must, therefore, first perform an initial synchronization in which, among other things, the phase and carrier frequency error must be found (acquisition) and continuously corrected (tracking).
In the receiver front end, a local oscillator is usually used for downconverting the received signal to an intermediate frequency or directly into the baseband. The local oscillator is usually implemented as VCO (voltage controlled oscillator) and is used for the transmitting and receiving operation. Problems are presented by switching-on or switching-over processes of transmitting and receiving operation (TX-RX or RX-TX) and corresponding switching processes at the VCO (or its load) which can result in considerable settling of the carrier frequency and phase (VCO glitch or VCO deviation) to the stable final value. These transients influence not only the synchronization preamble of a data packet but frequently continue far into the area of the payload. FIG. 1 shows by way of example the variation with time of the VCO frequency after a switching process (upper part) and the initial part of an OFDM burst (lower part). The OFDM burst has a so-called PLCP preamble with a length of 16 μs, known from the IEEE 802.11a standard. A first section of the PLCP preamble with a length of 8 μs is subdivided into ten short symbols, and a second section with a length of 8 μs consists of a guard interval and two OFDM symbols C1 and C2 which are used for channel estimation. These are followed by the payload symbols. In FIG. 1, it can be seen that the VCO transient reaches far into the area of the payload symbols. On WLAN cards available on the market (also IFX boards), frequency shifts of up to 16 kHz over up to 10 OFDM payload symbols were measured.
In OFDM demodulation, a VCO deviation in the frequency domain (post FFT) manifests itself as running away of the phase (common phase, CP). More detailed investigations show that this disturbs the payload to a much greater extent than the preamble synchronization which supplies good starting values for frequency and phases in spite of the VCO deviation. Without fast frequency/phase correction in the frequency domain (post FFT) during the first OFDM payload symbols (SIGNAL S followed by DATA D1, D2, . . . in WLAN), the phase coherence can be lost completely. In addition, considerable frequency shift leads to loss of the orthogonality and thus to intercarrier interference (ICI) of the received subcarriers. This effect can be combated effectively only in the time domain (pre FFT).
For the frequency/phase synchronization, pilot phase estimation (common phase estimation, CPE) and compensation (common phase correction, CPC), decision directed (DD) phase estimation and compensation and mixtures of both methods have previously been used. The pilot-based methods, i.e. methods based on the pilot subcarriers, are rugged and, above all, fast since they do not produce any decision errors and the phase errors can be corrected without delay in the same OFDM symbol. However, they suffer from higher noise effects since only a few pilots are available for the phase estimation (WLAN: K=4 pilots compared with 48 data carriers). The methods operating with decision-directed frequency/phase tracking, i.e. method based on the data subcarriers, are characterized by better estimating accuracy but, due to the decoding and remodulation delay (some OFDM symbols), are too slow to follow fast VCO phase changes. Such methods, which contain both pilot and decision-directed elements allow a certain tradeoff between ruggedness, speed and estimating accuracy, but are very complex.